Conventionally, β-amylase of plant origin, for example, β-amylase from soybeans, wheat, barley, malt, a sweet potato, and a potato has been known. Among them, β-amylase extracted and purified from grains such as soybeans, wheat, barley, and malt is industrially widely used for manufacturing, for example, maltose-containing syrup used in sugar production, bakery, and brewing industries. Among the β-amylase of plant origin, β-amylase derived from soybeans has a high enzymatic activity and also an excellent thermostability.
By the way, in recent years, due to the increase in demand for bioethanol, the price of corn has risen. Consequently, planting has been shifted from soybeans or wheat to corn. Therefore, soybeans, wheat, barley, and the like, are in short supply, and the prices thereof are rising. Under such circumstances, it is difficult to secure raw materials of β-amylase.
β-amylase is an enzyme that acts on polysaccharides such as starch and glycogen, which have the α-1,4 linkage of glucose as a main chain, and breaks down them in a maltose unit from the non-reducing end. β-amylase has traditionally been known to be found in higher plants such as soybeans and wheat. Since 1972 when it was reported that an enzyme exhibiting the action mechanism the same as that of higher plant β-amylase was present also in microorganisms, a large number of microorganisms have been found as β-amylase-producing microorganisms (Non-patent Document 1).
To date, Bacillus sp. such as Bacillus cereus, Bacillus polymyxa, Bacillus circulans, Bacillus megaterium, and Bacillus stearothermophilus, Streptomyces sp., Pseudomonas sp., and the like, have been reported as the β-amylase-producing microorganisms. However, most of them have low productivity, and few of them have been practically used.
On the other hand, amylase produced by filamentous fungi such as Aspergillus sp., breaks down amylose and amylopectin by the end type. Therefore, when the amylase of this type is used, glucose, maltotriose, and other oligosaccharides, in addition to maltose, are produced. Furthermore, the amylase of this type has a low thermostability and is less practical for production of maltose.
Bacillus stearothermophilus produces a maltose-producing enzyme having a high thermostability (see, Patent Document 1 and Non-Patent Document 2). This enzyme produces maltose by the exo type from the non-reducing end of starch, but maltose produced is α type. Furthermore, this enzyme does not hydrolyze strictly in a maltose unit as in β-amylase of plant origin. That is to say, it is reported that, in the initial time of reaction, in addition to maltotetraose (G4), maltotriose (G3) and maltose (G2), a small amount of maltopentaose (G5) and maltohexaose (G6) are also produced, and that this enzyme breaks down Shardinger dextrin into maltose and glucose, and breaks down maltotriose into maltose and glucose. As a result, in the starch decomposed product by this enzyme, 6 to 8% glucose is contained. Therefore, this enzyme is not suitable for manufacturing highly purified maltose syrups.